The subject technology relates to a method and apparatus for supporting a membrane for gas separation for use with a fuel cell and like devices. More specifically, a metal foil permselective membrane is supported over an open cavity by a planar microscreen element that includes a thin metal sheet form with a non-porous perimeter wall surrounding a perforated or porous central area that is sized to match the dimensions of the open cavity.
Fuel cells are electrochemical devices that produce direct current (DC) electricity by the reaction of a fuel with an oxidant, typically producing byproducts of heat and water. Common fuels are hydrogen, methanol, and carbon monoxide. The most common oxidant is oxygen, either in a relatively pure form or from ambient air. Fuel cells contain an anode, a cathode, and an electrolyte barrier between the anode and cathode. The fuel is introduced at the anode and the oxidant is introduced at the cathode. The electrolyte barrier, commonly referred to as a membrane-electrode assembly or MEA, is an ionically conductive thin barrier that is relatively impermeable to the fuel and oxidant, and is electrically insulating. Known fuel cell designs and operating principles are described in, for example, The Fuel Cell Handbook, 7th Edition (2004) published by the US Department of Energy, EG&G Technical Services under contract DE-AM26-99FT40575.
Various techniques are known for separating gases for use in chemical reactions in fuel cells and like devices requiring similar fuel. As known in the art, the physical and/or chemical properties of gases to be separated are exploited to separate them from a mixture of gases. Various methods such as adsorption, absorption, cryogenic distillation, permeation and the like can be used for separation of gases. For example, hydrogen can be separated from a gaseous mixture using a membrane/foil composed of palladium-copper alloys or palladium-silver alloys owing to the chemical property of hydrogen that makes it permeable through palladium-copper and palladium-silver alloys.
In the case of separation through permeation, membranes are used for separation of a gas from a mixture of gases. A gas can be separated by permeation because of selective permeability of the gas through the membrane based on the solution-diffusion mechanism. As the gas passes through the membrane, because of a pressure gradient on either side of the membrane, a difference in concentration of the gas between the two sides of the membrane is created. As a result, there is a net diffusion of the gas through the membrane from the high-pressure side to the low-pressure side.
Usually, the pressure gradient on either side of the membrane is at least 50 psi and often between 100 psi to 200 psi during operation. The membrane is also thin, sometimes very thin (e.g., from 0.003 inches thick to 0.0005 inches thick), since the flow of permeating gas through the membrane is inversely proportional to the membrane thickness. Thus, it is desirable to use a thin membrane to increase the permeate gas flow rate through the permselective membrane and to reduce membrane material volume in applications where the membrane material is costly, e.g., when the membrane comprises precious metals.
Generally the membrane is supported over one or more open cavities or passages so that permeate gas passing through the membrane has an unrestricted flow path. Heretofore, undesirably thick membranes were used to prevent the membrane from being deformed or punctured by pressure forces tending to force the membrane into the open cavities or passages. Additionally, gaps and/or discontinuities are often formed by the support structure so that the membrane deforms into these discontinuities and locations of stress are created. Such deformation of the membranes tinder pressure often causes rupture leading to failure. There have been many attempts to overcome these difficulties.
For example, U.S. Pat. No. 6,319,306 to Edlund et al. discloses a coarse mesh 74 for providing parallel flow conduits in FIG. 7. The coarse mesh 74 is sandwiched between fine screen members 76 to form a screen assembly 70. The fine screen members 76 minimize apertures and projections that may damage the membranes 46. The screen assembly 70 is placed in a frame 90 that has a central opening to make a permeate frame 91. The permeate frame 91 is covered with a gasket 92 that also has a corresponding central opening. The membrane 46 is overlaid onto the gasket 92 such that the membrane 46 deforms into the gasket opening to rest against the adjacent fine screen 76. Strain occurs where the membrane 46 transitions across the lip of the gasket's central opening. Such strain causes fatigue and rupture as noted above.
Additional patents yield similar drawbacks and/or complexity such as U.S. Pat. No. 7,033,641 to Saijo et al., U.S. Pat. No. 6,946,020 to Han et al.; U.S. Pat. No. 6,835,232 to Frost et al., U.S. Pat. No. 7,056,369 to Beisswenger et al. and U.S. Pat. No. 7,144,444 to Takatani et al.